The vena contracta stuff was interesting, thanks. Gas port erosion should be a factor here, more so on the forward edge. I read a Stoner interview somewhere that mentions this.
Also food for thought is your " v<Mach 1 through an orifice" point.
As a side point remember that the speed of sound in the combustion products will be much greater than that in our usual atmosphere, due to the high temperature. Unless the mean molecular weight is 10x higher, which I doubt.
“ridiculously high Reynolds number” is a great phrase. I’ll do some empirical tests and let Nature do the integrals using her analog methods.
Presuming these two items interrelate and also that you mean ‘high temperature’ associated with an engine burning an air+fuel mixture, then its not what is being described.
VE on an engine is how efficiently the intake charge can be drawn into the cylinder.
ie: ambient outdoor temperature air.
Venturi effect in an intake track can achive near supersonic speeds, and if actually met will cut flow to zero as the sonic wave more or less pushes back against any air moving in.
Supersonic cannot be exceeded because the wave cannot be overcome in normally asperated systems. Potentially attainable in forced induction systems however.
Of course if you actually meant combustion product and higher temperature as the rifle gas, then nevermind, I’m simply just being confused again
Yes, I meant the rifle gas. My point is that “supersonic flow” does not mean flow at greater than the standard speed of sound (1130 ft/sec or whatever), it means that the flow goes faster than sound goes in the medium that is flowing.
For example, the speed of sound in helium is more than 3000 ft/sec, so a flow of helium would have to be faster than this to be supersonic.
The combustion gases in the barrel during dwell time are very hot, so the speed of sound in these gases will be higher (sound speed goes about like the square root of temp in Kelvins).
But since the combustion products may have a higher molecular weight than the atmosphere, this would lower the speed of sound in these gases (the opposite of the helium comparison).
I greatly overestimated the temperature of combustion products when thinking about my post, so this effect may not be as big as I thought.
The velocity of the expanding gases is set to be a constant 4750 fps (approx) when uncorked. I believe when the bullet passes the port, the gases lose velocity going though the gas port. If I recall, the gases will drop to subsonic speeds. What subsonic is under these circumstances is, I do not know.
The gases passing through the port will also lose pressure and as they are entering a small expansion chamber, the gases will also throw off heat. Now it becomes a race between the slightly cooled, reduced pressure and velocity gases to reach the BCG before the bullet uncorks the barrel. The distance of the gas tube from gas block to BCG is longer than the distance from the port to the muzzle. The only way for the gases to win that race is to exceed the velocity of the bullet. What is the actual velocity of the gases traveling the gas tube and how long does it take to pressurize the BCG enough to get it moving?
Also, the Army did a study that shows the bullet has exited the muzzle several feet of an M14, which has it’s gas piston at the gas block, before the reciprocating parts have a chance to move.
Keep in mind, there is also a small delay while the pressure overcomes the inertia of the moving parts and the spring tension. I don’t have enough math to prove this out and can only base this conclusion on the Army tests. But the theory is sound. If you have facts that show otherwise, please share as I’m always interested in learning more
Mist, I wonder if the fact that the gas tube and all gas-system components already have gas inside of them may play a part in the ‘pressurization’ of the system. There’s going to be ambient air pressure inside, not a vacuum. Do you think this would have anything but a negligible effect? Just a WAG, as I have no background in this area.
Where did you get that number.
I have heard that gas goes up to 7x bullet velocity in free air, but that of course depends on the pressure and volume behind the bullet at the muzzle…
I did some research a couple of decades ago to see how much recoil was generated by the ejecta (the gases from the gunpowder). The formula I came across in one of my old reloading manuals for figuring out free recoil used 4750 fps (if I recall correctly- it may have been 4700 fps) as the constant for the velocity of the ejecta for commercially available smokeless powders. (I have also seen references in articles to the military using special powders with greater expansion rates for higher velocities in certain weapons.) If the muzzle velocity is low, say around 800 fps as it would be for a pistol round, the ejecta would be several times faster than the bullet.
I wanted to see how much recoil a 75 gr bullet fired at 4800 fps from a man portable rail gun would generate. Turns out it’s roughly the same as would be when firing the same bullet at 3200 fps from an M16A2 when both weapons weigh the same.
Also, I came across a post some time back that the author claimed the density of the gases in the barrel is about the same as balsa wood. I don’t know if the claim is correct, however
